The present invention relates to surface wave acoustic filter devices and more particularly the invention relates to the construction of such devices particularly with regard to features being useful for incorporating the device in a band-pass amplifier system.
Without attempting to restrict usefulness of the filter device in accordance with the present invention, surface acoustic wave filters are being employed, for example, in the i.f. channel of a TV set. The requirements posed here are quite stringent, particularly with regard to off-band rejection, and surface wave devices are well-suited for meeting these requirements. Such a surface acoustic wave device includes usually a flat substrate made of a piezoelectric material and carrying on one of its surfaces two or more pairs of interdigitized combs, which are established by means of metallization fingers on the substrate surface. These pairs constitute surface wave transducers, and they are aligned (in the surface) in the direction extending transversely to the direction of extension of the fingers. At least one of these transducers acts as transmitter for surface waves, at least another one acts as receiver. The wavelength of the surface waves produced and received is determined by the fingers spacing of the transducers. This finger spacing defines particularly the center frequency of the resulting filter pass-band range.
Off-band rejection is in the first instance defined and established by a technique called apodization. Apodization of at least one of the transducers provides for a particular contour in the finger overlap. It was found that this apodization contour permits the direct generation of a particular frequency response reducing off-band lobes below a tolerable level.
It was found, however, that such a device poses a number of other problems which tend to degrade its off-band rejection properties. One of the problems originates directly from the phenomena involved and utilized. A surface wave transducer inherently produces also bulk waves, i.e. the transducer transmits shear and compression waves into the interior of the piezoelectric substrate. These bulk waves will be reflected by any physical boundary or, more generally, by a discontinuity of acoustic wave transmissivity (acoustic impedance). Such a reflected wave will reach a receiving transducer in addition to the regular surface waves; it is for this reason that the reflected bulk waves degrade the off-band rejection properties of the filter. Moreover, the bulk waves may exhibit different transit times, because the propagation speed of Rayleigh waves is different from the propagation speed of compression and shear waves. In addition, bulk waves having pass-band frequency contribute to the production of the so-called pass-band ripple.
Another problem has to do with the reflection and transmission of surface waves. First of all, surface waves are readily reflected by the edges of the substrate, resulting in a direct echo that may be received by the receiving transducer. Another problem has to do with the specific interaction of transducer electrodes with the piezoelectric substrate underneath. A surface wave transducer can act as receiver as well as transmitter, because the interaction between the transducer electrodes and the piezoelectric material is a reversible one. Thus, a receiver may well act as transmitter and will, in fact, do so upon being energized as receiver. Particularly, a transducer upon receiving and responding to an incoming surface wave generates an electrical voltage, which, in turn, stimulates acoustically the piezoelectric material for generating surface waves. These waves if generated by the transducer which is supposed to be only a receiver, travel back to the original transmitter and interact therewith in like manner, whereupon surface waves are generated thereat which will reach again the receiver. Thus, the latter actually receives another signal which can also be termed an echo.
An echo signal produced as aforedescribed is delayed as compared with the original signal by a period of time equal to one round trip between the two transducers. Each signal (as to information content) travels actually three times the distance between the transducers so that this particular echo is called the triple transit echo. The several echo signals are disturbances which may become noticeable in a two-fold manner. First of all, they contribute to the so-called pass-band ripple and in addition they have the effect of true echoes. They arrived at the receiver but delayed relative to the original signal as received; in a TV circuit the echoes may produce so-called "ghosts".